Method of forming contacts to source and drains regions

Abstract: A method of semiconductor fabrication having applicability to forming contacts to sources and drains especially in SRAM applications is disclosed. A dielectric (e.g., 127) and an overlying polysilicon conductor (e.g., 131) are formed and patterned thereby exposing a semiconductor substrate (e.g., 123). A silicide layer (e.g., 132) is deposited, thereby contacting the polysilicon layer (e.g., 131) and the substrate (e.g., 123). Subsequent patterning of the silicide layer (e.g., 132) using an oxide hard mask provides electrical contact between the polysilicon layer (e.g., 131) and the substrate without the risk of trenching into the substrate (e.g., 123).

Description: Technical Field This invention relates generally to a method of making semiconductor integrated circuits and particularly to a method which forms contacts to sources or drains. Background of the Invention In the manufacture of semiconductor integrated circuits utilizing field effect transistors, it is often necessary to form electrical contacts to either the source or drain region. Various techniques and structures have been devised for forming contacts to source or drain regions. In the design of Static Random Access Memories (SRAMs) it is generally necessary to connect the source/drain of an access transistor to the gate of a pull-down transistor. For example, in FIG. 1, the junctions denoted by reference numerals 11 and 13 provide instances in which, for example, the gate of pull-down transistor 15 is connected to the source/drain of access transistor 19 or, the gate of pull-down transistor 17 is connected to the source/drain of access transistor 21. In conventional SRAM fabrication, one of the connections (11 or 15) is made at the so-called "poly-1" level. That is, the polysilicon which forms the gate of the respective pull-down transistor is patterned over the source/drain region of the access transistor, thereby forming a direct contact. The other access/pull-down connection is often made at a higher level of the integrated circuit (i.e., perhaps at the "poly-2" level or higher). FIGS. 2-5 illustrate a problem which occurs when a polysilicon gate is patterned directly over a source/drain. In FIG. 2, reference numeral 23 denotes a substrate which may be, typically, silicon, epitaxial silicon, doped silicon, etc. In general, the term substrate refers to a material or to a layer upon which other materials may be subsequently formed. Reference numeral 25 denotes a junction region which comprises the source or drain of an access transistor, such as transistors 19 or 21 in FIG. 1. Reference numeral 27 denotes a dielectric layer which, typically, may be silicon dioxide or silicon oxynitride. If layer 27 is silicon dioxide, it may typically have a thickness of approximately 10 nm (100Å). In FIG. 3, layer 27 has been patterned exposing surface 29 of junction region 25. In FIG. 4, polysilicon layer 31 has been deposited to cover exposed surface 29 of junction region 25. It is desired that polysilicon layer 31 should be patterned using a photoresist. Photoresist 33 is subsequently deposited over polysilicon layer 31 and patterned. It is desirable that edge 35 of photoresist 33 be immediately above (or, in FIG. 4, to the left of) edge 37 of patterned dielectric 27. Should edge 35 of photoresist 33 be slightly misaligned (and, as shown in FIG. 4, somewhat to the right of edge 37 of dielectric 27, i.e., should edge 35 overlie the opening defined by edge 37 of dielectric 27), subsequent exposure of polysilicon layer 31 to an etchant species may cause trenching into junction 25 of substrate 23. The trenching may damage the electrical characteristics of the junction and, under worst circumstances, may completely penetrate junction 25. Thus, an examination of FIGS. 4 and 5 shows that, if patterned edge 35 of resist 33 is positioned parallel with, or to the left of, patterned edge 37 of dielectric 27 (i.e., edge 35 overlies layer 27), subsequent etching will stop on dielectric 27 and not damage junction 25. However, should patterned edge 35 or photoresist 33 be to the right of patterned edge 37 of dielectric 27, the trench 39 depicted in FIG. 5 may be obtained. EP-A-0192093 discloses a semiconductor device including a P-N diode integrated with some field effect transistors. A portion of a first polysilicon layer which provides the gate electrode of one of the transistors is patterned to have an edge at the edge of the substrate portion in which the N-region of the diode is to be formed. A second polysilicon layer is formed, contacting the first polysilicon layer and the substrate portion and continuing beyond the substgrate portion to contact a source/drain region of another of the transistors. EP-A-0415528 discloses an integrated circuit in which polysilicon conductors are formed, including a gate electrode of a FET and pads to be connected to the source/drain regions of the FET. The polysilicon conductors are provided with insulating sidewall spacers. Silicide-forming metal is blanket deposited and an amorphous silicon layer is deposited over the metal and patterned to form strips overlying the polysilicon pads and extending to the regions to which the pads are to be connected. The metal is then reacted with the strips and the substrate, so that silicide is formed where the metal contacts the substrate, i.e. the source-drain regions, and where it is covered by the strips. Unreacted metal is then removed. Summary of the Invention A method according to the invention is set out in claim 1, preferred forms being set out in the dependent claims. Detailed Description FIG. 1 is a circuit diagram illustrating a typical SRAM; FIGS. 2-5 are cross-sectional views which illustrate a problem inherent in semiconductor fabrication processes; and FIGS. 6-18 are cross-sectional views useful in understanding an illustrative embodiment of the present invention. Detailed Description In FIG. 6, reference numeral 123 designates a substrate which may typically be silicon, epitaxial silicon, or doped silicon. Doped region 125 may be formed, for example, by diffusion or ion implantation. Alternatively, doped region 125 may be formed at a later stage in the process by driving dopants out from subsequently formed silicide layers. For convenience in visualization, doped region 125 is depicted at the early stage of the inventive process. Layer 127 is a dielectric layer, typically silicon dioxide, having a thickness of approximately 10 nm (100Å) or, possibly, silicon oxynitride. Layer 131 is desirably a conductor such as polysilicon, typically having a thickness of approximately 100 nm (1000Å). Illustratively, layers 127 and 131 are formed during the early stages of semiconductor integrated circuit processing. Both layers 127 and 131 may, in other parts of the integrated circuit, comprise respectively a gate oxide and gate conductor. Turning to FIG. 7, layers 127 and 131 have been patterned, exposing a portion of the surface of substrate 123. It is desired to make a patterned electrical connection between layer 131 and the exposed surface. In FIG. 8, blanket layers of a refractory metal silicide, for example, tungsten silicide 132, and a dielectric 134, for example, plasma-enhanced TEOS or BPTEOS may be deposited. If desired, before (or even after) formation of layer 134, silicide 132 may be doped, perhaps by ion implantation. An anneal may be performed to drive the dopants from silicide 132 into substrate 123, thereby forming junction region 125 (which has been depicted since FIG. 6 for conceptual convenience). It is desired to pattern layers 132 and 134 so that the patterned edges of these dielectrics are co-linear with (or to the left of) the edges of dielectrics 131 and 127. That is, turning to FIG. 9, after photoresist 135 is deposited and spun, photoresist 135 is patterned creating edge 136. Ideally, edge 136 is co-linear with edge 137 of layers 131 and 127, respectively. Should edge 136 be co-linear with edge 137, turning to FIG. 10, layer 134 may be etched to create a hard mask and, subsequently, layer 132 may be etched with the aid of the hard mask to create patterned conductor with patterned edge 139 which precisely abuts edge 137 of layers 131 and 127. Returning to FIG. 9, unfortunately, it is not always possible, because of alignment tolerances, to pattern photoresist 135 so that edge 136 is precisely co-linear with edge 137 of layers 131 and 127. Sometimes, edge 136 will be to the left of edge 137; on other occasions, edge 136 may be positioned to the right of edge 137. In FIG. 9, edge 136 may be described as overlying edge 137, which defines an opening over substrate 123. FIGS. 11 and 12 illustrate what happens when edge 136 of photoresist 135 is positioned, because of misalignment tolerances, to the left of edge 137. In other words, patterned edge 136 of photoresist 135 does not overlie the opening created by edge 137 over substrate 123. Examining FIGS. 11 and 12, after the patterning of resist 135, an anisotropic etch is performed, thereby forming the configuration depicted in FIG. 12. It will be noted that polysilicon layer 131 is etched together with silicide layer 132 and dielectric 134. Subsequent processing may be employed to remove dielectric 134, if desired. An alternative method of etching (proceeding from FIG. 11) is to etch a hard mask into dielectric 134 and then, employing the hard-mask technique, etch underlying layers 132 and 131. It will be noted that, in the situations depicted in FIGS. 9-10 and 11-12 (i.e., those situations in which the edge of the photoresist mask is outside or coincident with the edge of the opening defined by layers 127 and 131), that no problem will occur, even if more conventional processing techniques described in FIGS. 2-5 are employed. However, should the defining edge 136 of photoresist 134 overlie the openings defined by edge 137 of layers 127 and 131, use of the technique described in FIGS. 2-5 cannot eliminate a risk of trenching. However, applicants' inventive procedure helps to avoid trenching, provided the misalignment is not too great. In FIG. 13, it will be noted that edge 136 of photoresist 135, is slightly interior to edges 137. That is, edge 136 overlies the opening over substrate 123 defined by edge 137. In FIG. 13, edge 136 overlies the vertical sidewall 150 of silicide layer 132. After etching of layers 134 and 132 is completed, the configuration depicted in FIG. 14 is obtained. It will be noted, examination of FIG. 14 shows that the sidewall portion 150 of silicide layer 134 has protected substrate 123 against trenching (unlike the case depicted in FIG. 5). Etching of layers 134 and 132 produces a flat surface 151 in silicide 134 which is roughly parallel with upper surface 153 of polysilicon layer 131. In FIG. 15, further etching has been performed to remove that portion of polysilicon layer 131 at the left-most part of the figure and to remove dielectric 134. Again, examination of FIG. 15 shows that the presence of sidewall 150 has prevented trenching of the substrate. Slightly greater misalignments than that depicted in FIGS. 13-15 may also be tolerated. FIGS. 16, 17, and 18 depict an example of somewhat greater misalignment. In FIG. 16, edge 136 of resist 135 is positioned interior to edge 137. Furthermore, edge 136 of resist 135 is interior to edge 150 of silicide layer 132. In FIG. 17, layers 134 and 132 are etched (with or without hard-mask techniques). Finally, in FIG. 18, the exposed portion of polysilicon layer 131 has been removed, leaving oxide layer 127. It will be noted that silicide layer 132 is etched during the removal of polysilicon layer 131. Should (returning to FIG. 16) edge 136 of photoresist 135 be positioned too far to the right, i.e., too far to the interior of the opening defined by edge 137, there is some danger that in the structure ultimately formed and depicted in FIG. 18, that cusp 151 adjacent fillet 153 may touch, or in worst circumstances, penetrate the substrate.

Claims: A method of forming a contact to a source or drain portion (125) of a substrate (123) in a semiconductor integrated circuit including field effect transistors comprising: forming a patterned dielectric (127) over said substrate, said patterned dielectric exposing said substrate portion; forming a patterned first conductor (131) over said patterned dielectric, said patterned first conductor having an edge (137) at the edge of said portion and not contacting said substrate; forming a second conductor layer (132) contacting said patterned first conductor, including said edge, and contacting said substrate portion; forming a material layer (134) over said second conductor; CHARACTERISED BY: patterning said material layer, thereby creating an edge (139) of said material layer approximately overlying said edge (137) of the first conductor at the edge of said portion and exposing a portion of said second conductor layer; etching the exposed portion of said second conductor layer (132), whereby the second conductor layer or the second conductor layer and the material layer covering said edge (137) of said first conductor (131) protects said substrate portion during said etching in the event of misalignment between said edge (139) of the material layer and said edge (137) of the first conductor. The method of claim 1 in which said first conductor is polysilicon. The method of claim 1 in which said second conductor (132) is a refractory metal silicide. The method of claim 3 in which said refractory metal silicide is tungsten silicide. The method of claim 1 in which said material layer (134) is silicon dioxide. The method of claim 5 in which said silicon dioxide is formed from TEOS. The method of claim 1 in which said step of patterning said material layer creates an edge which overlies the previously exposed portion of said substrate. The method of claim 7 in which said second conductor (132) is conformal and has a first thickness covering a substantial portion of the previously exposed portion of said substrate and has a second thickness, said second thickness being greater than said first thickness, adjacent said patterned dielectric and said patterned first conductor (131) and in which said edge of said material layer overlies that portion of said second conductor having said second thickness.

Inventors: LEE KUO-HUA, YU CHEN-HUA DOUGLAS, LEE, KUO-HUA, YU, CHEN-HUA DOUGLAS
Applicants: AT & T CORP, AT&T CORP.
Application year: 1994
Main classification: H01L21
IPC codes: H01L21, H01L27